The human immunodeficiency virus (HIV) types 1 and 2 and the simian immunodeficiency virus (SIV) are members of the lentivirus subfamily of the Retroviridae. They contain three genes (gag, pol, and env) that are present in all retroviruses and five (nef, rev, tat, vif, and vpr) that in combination are only present in HIV and SIV. HIV-1 also contains vpu that is not present in SIV or HIV-2. The latter six genes are referred to as accessory genes. However, this nomenclature is likely to minimize their importance because these proteins are essential virulence factors. The topic of this grant, nef, is found only in HIV and SIV where it is important for efficient virus replication and the eventual development of the pathology associated with AIDS in humans and primates. Therefore, though the biological significance of Nef has been proven, the exact function of Nef at the cellular level is not known. Nef alters intracellular trafficking of CD4 and MHC I. Nef also has a positive effect of virus infectivity. In addition, Nef alters signaling pathways in virus-infected cells via its ability to activate Pak2. Although the molecular basis for these phenotypes remain to be fully elucidated, several important observations have been made that highlight the complex interplay between HIV and the infected cell. Nef might not be directly responsible for the development of the pathology associated with HIV disease but rather for the enhanced virus replication which results in the deterioration of the immune system of the infected host. One of the main challenges lying ahead is to correlate in vitro functions with the phenotypes observed in vivo. Progress made towards a better understanding of Nef function and its molecular determinants will facilitate the rational design of inhibitors of Nef function and of virus replication. Therefore, this renewal is focused on the molecular basis and consequences of the activation of Pak2 by Nef: 1) To determine the molecular determinants of Pak2 activation by Nef, 2) To determine the role of Rho GTPases in the activation of Pak2 by Nef, 3) To determine the functional consequences of the activation of Pak2 by Nef, and 4) to determine the role of Pak2 in HIV infection. [unreadable] [unreadable]